It is well known that it is desirable to measure the temperature of the melt throughout the length of a nozzle on a hot runner injection molding machine, and at the gate into a mold cavity.
Several attempts at taking this measurement have been made. Typically, a thermocouple is included on the nozzle and is mounted to the exterior of the nozzle body. In order to take measurements that better represent the condition of the melt, the tip of the thermocouple is usually positioned within an aperture that penetrates into the nozzle body so that the tip of the thermocouple is positioned nearer to the nozzle melt channel. The accuracy of the thermocouple is hampered, however, by the proximity of the thermocouple to the nozzle heater, which is typically positioned on the exterior of the nozzle. Thus, the proximity of the thermocouple to the nozzle heater itself prevents the thermocouple from accurately measuring the temperature of the melt.
Another example of an attempt to measure the melt temperature at the gate is disclosed in European Patent Application EP 99304442.9 (Goldwin et al.). Goldwin et al. discloses the use of a conductive film to coat the outside of a valve pin that passes through the nozzle melt channel. The conductive film could be used to measure the temperature of the melt in the nozzle melt channel. However, the film is repeatedly exposed to a cycling of pressures, and is constantly abraded by the melt flowing through the nozzle into the mold cavity.
Yet another example of an attempt to measure the melt temperature at the gate is disclosed in U.S. Pat. No. 5,334,008 (Gellert). Gellert discloses a thermocouple, having a sensing portion that is fixed inside a valve pin guiding element in a melt channel. The guiding element divides the melt flow, however, and creates an obstruction in the melt channel. Furthermore, the thermocouple is fixed within the melt channel, and cannot therefore obtain temperatures from different positions within the melt channel.
For some applications, it may be advantageous to measure a plurality of temperatures. For example, in some co-injection applications, where there are flows of more than one melts into a mold cavity, it may be desirable to measure the temperatures of some of the melts individually, and/or some of the melts after they have combined. In order to achieve this using fixed thermocouples of the prior art, a plurality of thermocouples may be needed to be incorporated into the co-injection nozzle. In the event that one of the thermocouples fails for any reason, it can be relatively difficult to access the failed thermocouple to replace it.
Thus a need exists for new devices for the measuring of the temperature of the melt at the gate into a mold cavity in a hot runner injection molding machine.